1. Field of the Invention
This invention relates to compact zoom lenses, and more particularly to a zoom lens of reduced bulk and size with improved correction of aberrations throughout the zooming range by using an aspherical surface in one of relatively movable lens units.
2. Description of the Prior Art
Recently, small-sized light-weighted high performance compact zoom lenses have been demanded for photographic cameras, video cameras, and TV cameras.
To achieve a reduction of the total length and the outer diameter of the zoom lens and an improvement of the correction of aberrations throughout the entire zooming range, it is required that the refractive power of each of the lens units be strengthened and that an optimum power distribution has to be determined. Since strengthening of the refractive power generally teads to increase the various aberrations, it becomes very difficult to correct the aberration during zooming. Another method of reducing the bulk and size of the zoom lens is to increase the number of lens units movable for zooming with an advantage of facilitating an improvement of the correction of an aberrations throughout the zooming range. An example of an application of this method has been proposed in Japanese Laid-Open Patent Applications Nos. Sho 57-168209 and 57-169716 where the zoom lens comprises, from front to rear, a first lens unit of positive power, a second lens unit of negative power, a third lens unit of positive power and a fourth lens unit of positive power, all of which are made movable for zooming, with the use of a refractive power distribution that the on-axial rays of light emerging from the third lens unit are almost parallel. This refractive power distribution has, however, been found to be associated with a limitation on advance in the compactness of the zoom lens. Then, an alternative power distribution may be considered that the rays of light emerging from the third lens unit are convergent. The use of this power distribution, though providing the possibility of achieving a further reduction of the total length of the zoom lens, tends to increase the difficulty of removing the variation of aberrations with zooming for a high grade of imaging quality.
The zoom lenses of such type when constructed with the lens elements all of which have the spherical surface and corrected for various aberrations in each of the lens units, have generally an increased number of lens elements in each lens unit with the result that the total length and the weight tend to increase. When the bulk and size of the zoom lens is reduced by strengthening the refractive power of, for example, the third lens unit, the spherical aberration and astigmatism are increased within the third lens unit, becoming difficult to correct by a suitable design of the other lens units. Particularly, when an approach to the reduction of the range of variation of aberrations with zooming by correcting the aberrations of the lens units other than the third lens unit is employed, the power distribution becomes so improper that good correction of aberrations becomes difficult to be maintained throughout the zooming range.
An attempt has been made to enable the third lens unit to produce correction of the aberrations by using a diverging lens surface therein. But, this method has a disadvantage of unavoidably using an additional positive lens, sacrificing the reduction of the physical length of the lens system. Another disadvantage is the difficulty of well correcting the higher order aberrations produced from the strong diverging surface.